42-16 Hydrochemical Evolution of Glacier-Fed Watersheds in the Cordillera Blanca, Peru

Abstract:

Accelerated glacial retreat in the Cordillera Blanca of Peru is altering watershed hydrochemistry by exposing sulfide-rich bedrock to weathering (1). This study investigates the impact of acid rock drainage on water quality in the glacier-fed Quilcay River valley and compares these waters to synoptic samples collected throughout the larger Santa River watershed. Field measurements and laboratory analyses using ICP-MS and ICP-OES reveal highly acidic waters within the valley (pH ~1.5–5.5), driving elevated concentrations of Fe, Mn, Al, and trace metals including Co, Ni, Zn, and Cu. Rare earth elements (REE), including Dy, Er, Eu, Gd, La, Nd, and Sm, systematically increase with decreasing pH and display coherent fractionation patterns consistent with acid-driven mineral dissolution. Strong correlations between these elements and decreasing pH indicate sulfide oxidation as the dominant geochemical control. Despite elevated Ca and Mg concentrations, carbonate buffering is kinetically overwhelmed by proton production, resulting in a reaction-rate controlled system. Comparison with data from filtered samples collected previously (2008-2013) demonstrates persistent elevation-controlled trends but suggests a downstream shift in peak trace metal mobilization (1, 2). Quilcay valley waters are chemically more aggressive than the Santa River synoptic samples, reflecting strong glacial influence and limited dilution. These results indicate the presence of distinct hydrochemical regimes and highlight the long-term implications of glacial retreat for downstream water quality and ecosystem health.

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